Abstract— The desired product of the tequila fermentation process is ethanol. However, there are factors that may inhibit the alcohol production such as high substrate and ethanol concentrations present in the tequila must or culture medium. A model for predicting alcoholic fermentation behavior would be a valuable instrument for tequila research, due to the technical and economical implications. Therefore, an unstructured kinetic mathematical model taking into account substrate and product inhibition was proposed to predict tequila batch fermentation behavior. Several kinetic models were evaluated; the combination of the Moser and Luong kinetic model gave the best prediction. The nonlinear mathematical model performed satisfactory on biomass, substrate, and ethanol predictions. Keywords— Fermentation, kinetic model, substrate and product inhibition, tequila. I. INTRODUCTION HE fermentation stage is crucial in the production of alcoholic beverages because in this stage the ethanol is produced. Nevertheless, the fermentation is affected by environmental, physicochemical, and biological factors such as temperature, pH, dissolved oxygen, and growth inhibitors, among others [1]. In a fermentative process it is desired to maximize the production of a desired metabolite, however, without the accurate knowledge of the process this can not be attained. Modeling a fermentation process presents some advantages such as process knowledge improvement, decreasing the cost of expensive industrial experimentation [2] mathematical optimization [3] and process control [4]. Tequila is the most important distilled alcoholic beverage in México. It is produced from the fermented juice of cooked Agave tequilana Weber (blue variety). The tequila process involves multiple steps: fermentable sugars are obtained by Manuscript received January 9, 2007; revised May 23, 2007. This work was supported by the Mexican Consejo Nacional de Ciencia y Tecnología under Grants 41148 and 24547. M. Arellano-Plaza is with Centro de Investigación y Asistencia en Tecnología y Diseño del Estado de Jalisco A.C., Av. Normalistas 800, Colinas de la normal, Guadalajara Jalisco, México. (phone: 52-3333455200, fax: 52- 3333455200; e-mail: marellano@ciatej.net.mx ). E. J. Herrera-López, D. M. Díaz-Montaño, J. J. Ramírez-Córdova and A. Moran are with Centro de Investigación y Asistencia en Tecnología y Diseño del Estado de Jalisco A.C., Av. Normalistas 800, Colinas de la normal, Guadalajara Jalisco, México. (e-mail: eherrera@ciatej.net.mx , dmdm@ciatej.net.mx , jramirez@ciatej.net.mx ) steaming, milling, and pressing the agave head plants. During the steam cooking process, the polyfructans are hydrolyzed into a mixture of sugars which mainly consist of fructose. In some tequila distilleries, the fermentation occurs spontaneously while in others, the agave juice is inoculated using commercial or indigenous yeast cultures, often Saccharomyces cerevisiae species. After fermentation, the must is distilled twice and the final product is diluted to an alcohol content of 35 to 38% v/v. Maturation is carried out by storing the tequila in white oak barrels from two to 11 months to obtain “tequila reposado”, for more than 12 months is called “tequila añejo”, and beyond three years is known as “tequila extra-añejo” [5]. The tequila production is strictly regulated by the Mexican government by means of the Norma Oficial Mexicana del tequila [6]. In the tequila fermentation process ethanol and many volatile compounds that may influence the sensory characteristics of the final product are formed. Some compounds like ethanol are toxic to microorganisms and may cause product inhibition. Also, sluggish fermentation is a frequent problem in tequila distilleries. This problem is due to the lack of sugar consumption by yeast, mainly caused by stress conditions promoting an adverse environment. During wine alcoholic fermentation, the factors that may affect the microbial performance are: high sugar concentrations (osmosis stress), micro nutrient limitations (nitrogen and phosphate uptake), high ethanol concentration (produced during fermentation), low pH, low oxygen levels, poor mixing, extreme temperature, toxic substances (present on the must like sulphite, agro chemistry waste, killer toxins), and wild microorganisms not inoculated [7], [8]. Thatipamala reported that the most important cause of sluggish fermentations is the substrate and product concentration [9]. The effect of ethanol and sugar concentrations on the ethanol production is crucial in the tequila fermentation process. Therefore, the main aim of this work was to produce an unstructured mathematic model to predict the substrate and product inhibition on the tequila fermentation process. This model will allow better knowledge of the tequila fermentation process. A mathematical model which predicts alcoholic fermentation behavior will be a valuable instrument for tequila research due to the technical and economical implications. Unstructured Kinetic Model for Tequila Batch Fermentation M. Arellano-Plaza, E. J. Herrera-López, D. M. Díaz-Montaño, A. Moran, and J. J. Ramírez-Córdova T INTERNATIONAL JOURNAL of MATHEMATICS AND COMPUTERS IN SIMULATION Issue 1, Volume 1, 2007 1